1. Field of the Invention
This invention pertains to the generation of coherent electromagnetic radiation by the frequency conversion of laser radiation. In particular, this invention utilizes higher order vibrational or rotational Raman scattering to obtain efficient frequency conversion.
2. Description of the Prior Art
Frequency conversion of laser beams by means of vibrational or rotational Raman scattering is well known. See, for example, N. Gloembergen, "The Stimulated Raman Effect," Amer. J. of Phys. 35, 11, 983 (Nov. 1967), and A. Z. Grasyuk, "Raman Lasers," Sov. J. Q. Electr, 4, 3, 269 (Sept. 1974). High efficiencies of the order of 60% have been obtained for the conversion of laser beams to frequencies offset from the original laser frequency by an amount corresponding to the vibrational frequency of the molecular species utilized in the Raman cell (utilizing conversion into the first Stokes order). Quantum efficiencies as high as 90% were obtained by Grasyuk for the conversion of laser energy into the first Stokes order. A. Z. Grazyuk, "Raman Lasers," Sov. J. Quant. Elect., 4, 3, 276 (Sept. 1974). However, Grasyuk selected amplifier operating conditions so as to avoid the conversion of energy into Stokes components higher than the first. As a consequence, the translation in frequency by the Stokes device was limited to the vibrational frequency of the molecular species utilized in the Raman cell.
Larger translations in the frequency of the converted radiation have been obtained by use of higher order Raman scattering where the generated frequency is offset from the laser frequency by several times the vibrational frequency of the molecule (n-times for the n'th Stokes order), but the conversion efficiencies obtained in the prior art are low. See, e.g. T. R. Loree, R. C. Sze, and D. L. Barker, "Efficient Raman Shifting of ArF and KrF Laser Wavelengths," Appl. Phys. Letter, 31, 1, 37 (July 77).
Very large translations in frequency to the microwave region of the spectrum with high efficiency have been described using two laser beams of different frequencies mixing within a magneto-gas plasma. See, e.g. U.S. Pat. No. 3,546,477. However, the latter conversion device emits radiation primarily in the microwave region of the spectrum rather than in the optical region.